1. Technical Field
The present exemplary embodiments relate to a magnetic resonance imaging (MRI) apparatus which generates an image by showing a flow portion such as a blood vessel in which a fluid flows clearer than a static portion or by showing a tissue different in susceptibility from a normal tissue clearer than a normal tissue.
2. Related Art
A magnetic resonance imaging method for arteries and veins, that is, MR angiography (MRA) includes a time of flight (TOF) method using a gradient echo (GRE) method, and a black-blood (BB) method using a fast spin echo (FSE) method for imaging a blood vessel at low signal intensity. Recently, a susceptibility-weighted imaging (SWI) method which applies the susceptibility effect of veins has become available.
A non-contrast TOF method is a typical example of a white-blood (WB) method. The non-contrast TOF method utilizes an in-flow effect, so that an artery with a high flow velocity close to an inflow part of a slab has high signal intensity. In this non-contrast TOF method, it is difficult to visualize turbulent parts, and peripheral blood vessels such as perforating branches are not easily visualized, thus arteries are principally visualized. When an image is taken with a T1-weighted (T1W) sequence using a paramagnetic contrast agent, blood vessels are visualized at high signal intensity, which means a WB method. In addition, an MRA method in which blood vessels show higher signal intensity than background tissues is generally referred to as the WB method here.
In the BB method, blood vessels show lower signal intensity than peripheral tissues, slow blood flows are also visualized, and blood vessel walls are correctly visualized. It is also possible in the BB method to visualize the turbulent parts which are difficult to visualize in the TOF method. The sequence of the BB method was initially developed by using the FSE method, but is not used very widely due to the problem of image processing or other. In the BB method, while both arterial blood and venous blood show low signal intensity, arteries can be emphasized by setting a slightly shorter echo time (TE). In addition, when an image is taken with a T2*-weighted (T2*W) based sequence using the paramagnetic contrast agent, blood vessels are visualized at low signal intensity, which means the BB method.
In the BB method, peripheral tissues show low D signal intensity, and it is therefore difficult to separately extract the blood vessels alone. For example, it is difficult to exclude air by minimum intensity projection (mimIP) in the BB method. The blood vessels can be relatively easily extracted in the WB method by, for example, maximum intensity projection (MIP).
Another known MRA method is a phase contrast method. The phase contrast method achieves imaging by using the amplitudes and phases of two sets of signals which have been collected after a gradient magnetic field is used as a bipolar gradient so that the polarities of these signals are the reverse of each other.
While MRA is an imaging method for obtaining an image in which a flow portion and a static portion are shown with a contrast therebetween, there is also known an imaging method different from the MRA which obtains an image that shows the difference of susceptibility as a contrast. For example, an imaging method is known which obtains an image that shows an abnormal tissue such as a bleeding tissue and normal tissues around the abnormal tissue with a contrast therebetween.
Various methods as described above have heretofore been known to show the flow portion and the static portion or the abnormal tissue and the normal tissue with a contrast therebetween. However, for accurate or efficient medical diagnoses, there has been a demand for an image that provides a greater contrast to show the flow portion or the abnormal tissue more clearly.
Moreover, a technique described in the specification of U.S. Pat. No. 6,501,272 is capable of bringing the signal value for the inside of a blood vessel closer to zero, but is limited in that it cannot produce a negative signal value. This technique also entails complicated processing and a decreased signal-to-noise ratio (SNR).
In the phase contrast method, magnetic resonance signals have to be collected in two sets of sequences to obtain one image. This leads to a longer imaging time. Moreover, as the phase difference is limited to 180 degrees, the velocity of a target blood flow has to be known, and it is difficult to set an appropriate imaging parameter to obtain a satisfactory image.
Under these circumstances, the present applicant has proposed, as Jpn. Pat. Appln. KOKAI Publication No. 2008-272248 (US2008/0119721), a technique for generating, on the basis of data obtained by the WB method and data obtained by the BB method, another type of data which provides a higher contrast between a tissue of interest and a background than the data obtained by the above-mentioned methods. In accordance with the principle of this technique, a signal value obtained by the BB method is subtracted from a signal value obtained by the WB method. Thus, as the difference between the signal value obtained by the WB method and the signal value obtained by the BE method is greater in a blood vessel than in a background portion, it is possible to obtain data in which the difference between the signal value of the blood vessel and the signal value of the static portion is greater than in both the data obtained by the WB method and the data obtained by the BB method.
However, in image reconstruction by MRA, information on the amplitudes of the magnetic resonance signals alone has been conventionally used. Thus, in the BB method, a thick blood vessel with a high flow velocity of blood, for example, may not be completely dephased and may be returned from a negative to a positive when the signal value of a part with a negative phase is an absolute value. In this case, the contrast rather decreases if the technique described in Jpn. Pat. Appln. KOKAI Publication No. 2008-272248 is applied.
Furthermore, when the background portion has a signal void in the BB method, the signal value of the blood vessel is higher than the signal value of the background portion due to the above-mentioned return. Thus, the contrast considerably decreases if the technique described in Jpn. Pat. Appln. KOKAI Publication No. 2008-272248 is applied.
On the other hand, a GRE sequence is generally used in the TOF method for obtaining a WB image. In the TOF method, a rephase sequence is generally used so that spins in a voxel may be in phase to produce a vector sum for maximizing a signal. The rephase sequence is normally obtained by primary gradient moment nulling (GMN). In the primary GMN, the phases of zeroth and primary flow components which are predominant in a magnetic resonance signal should be substantially zero, thus the information on the amplitudes of the magnetic resonance signals alone has been conventionally used for image generation by the TOF method.
However, a moment of a second order or higher order is not rephased in primary GMN. Therefore, spins in a voxel are not completely in phase, and no magnetic resonance signal having the maximum amplitude component is obtained. However, in GMN, variation patterns of a gradient magnetic field pulse are more complicated and TE increases if moments of higher orders are rephased. Thus, primary GMN has heretofore been generally used as described above. A GRE sequence of zeroth GMN may be used to further reduce the TE. In accordance with zeroth GMN, the diffusion of phases in a voxel does not increase much and the capability of visualizing turbulent parts such as an aneurysm may be improved owing to the reduction of components of a second moment or higher order moment attributed to the TE reduction effect. However, it is pointed out that the capability of visualization in a periphery equivalent to a major arterial secondary branch or farther may decrease.
As described above, a sufficient contrast may not be obtained in the WB method as well.
The same holds true not only with blood vessel imaging but also with an imaging method which visualizes the abnormal tissue by use of the difference n susceptibility between the normal tissue and the abnormal tissue.